The general field of the present disclosure relates to protective coatings applied to metals. More specifically, the invention is directed to methods for smoothing the surfaces of such coatings.
Thermal barrier coatings (TBC's) are often used to improve the efficiency and performance of metal parts which are exposed to high temperatures. Aircraft engines and land-based turbines are made from such parts. The combustion gas temperatures present in turbines are maintained as high as possible for operating efficiency. Turbine blades and other elements of the engine are usually made of alloys which can resist the high temperature environment, e.g., superalloys, which have an operating temperature limit of about 1000.degree. C.-1150.degree. C. Operation above these temperatures may cause the various turbine elements to fail and damage the engine.
The thermal barrier coatings effectively increase the operating temperature of the turbine by maintaining or reducing the surface temperature of the alloys used to form the various engine components. Most thermal barrier coatings are ceramic-based, e.g., based on a material like zirconia (zirconium oxide), which is usually chemically stabilized with another material such as yttria. For a turbine, the coatings are applied to various surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles. Usually, the thermal barrier coating ceramics are applied to an intervening, metallic bond layer which has been applied directly to the surface of the metal part.
The thermal barrier coatings are often applied to the part by a thermal spray technique, such as a plasma spray process. In this technique, an electric arc is typically used to heat various gasses, such as air, oxygen, nitrogen, argon, helium, or hydrogen, to temperatures of about 8000.degree. C. or greater. (When the process is carried out in an air environment, it is often referred to as air plasma spray or "APS".) The gasses are expelled from an annulus at high velocity, creating a characteristic thermal plume. Powder material (e.g., the zirconia-based composition) is fed into the plume, and the particles are melted and accelerated toward the substrate being coated. For some applications, plasma-spray techniques have numerous advantages over other coating techniques, such as electron beam physical vapor deposition (EB-PVD). As an example, plasma spray systems are usually less costly than EB-PVD. Moreover, they are well suited for coating large, complex-shaped parts, with good control over the thickness and uniformity of the coatings.
While plasma-spraying is often the best technique for applying thermal barrier coatings, use of the process can present some problems under various circumstances. For example, a plasma-sprayed coating often has a relatively rough surface, e.g., an "Ra" (center-line average roughness) value greater than about 8 microns. Much smoother surfaces are required when the coating is to be applied to turbine components like airfoils, so that the convective component of the heat flux delivered to the coating can be reduced. Moreover, a smooth surface can also reduce aerodynamic drag losses.
A number of techniques can be used to smooth the surface of coatings like TBC's. Examples include grinding, tumbling, heavy-sanding, and heavy-polishing operations. In some situations, these techniques are quite effective. However, they can also be very time-consuming (especially with large, complex-shaped parts), adding considerably to the overall cost of fabrication. Moreover, they can sometimes mechanically damage the thermal barrier coating. For example, a sand-tumbling operation can sometimes result in the preferential smoothing/wearing of certain areas of the coating. The decreased thickness in those areas can undesirably lower the thermal resistance of the thermal barrier coating. Moreover, grinding and some of the other operations can sometimes result in the formation of pits, due to the "pull-out" of local portions of the coating material.
Polishing operations can often be effective for smoothing a protective coating surface, and will not have as much of a tendency to damage the surface, as in the case of some of the other operations. However, heavy-polishing (which might be considered as "grinding") can still result in the unequal smoothing or wearing of the coating surface. Moreover, the polishing of certain types of coatings, such as air plasma-applied zirconia-based TBC's, can still result in pits (i.e., from pull-out) in the surface. Furthermore, large cracks are sometimes formed in relatively thick coatings deposited by plasma spray techniques. It is often not possible to remove these types of cracks by the heavy polishing operations. The pits and large, deposition-related cracks are often unacceptable features on the surface, because they increase the hot gas turbulence, causing increased heat transfer and reduced thermal protection of the base metal.
It should therefore be apparent that new methods for smoothing the surface of a protective coating like a TBC would be welcome in the art. The methods should smooth the surface to a degree suitable for aerodynamic applications, while maintaining all of the beneficial characteristics of the protective coating. It would sometimes be desirable if the methods also assisted in controlling the thickness of the protective coatings. Moreover, the methods should be fully compatible with the application of the thermal barrier coating over a substrate, and should not add excessive cost or time to the overall production operation.